Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

基于碳化硅衬底的宽禁带半导体外延

宽禁带半导体具备禁带宽度大、电子饱和飘移速度高、击穿场强大等优势,是制备高功率密度、高频率、低损耗电子器件的理想材料。碳化硅(SiC)材料具有热导率高、化学稳定性好、耐高温等优点,在SiC衬底上外延宽禁带半导体材料,对充分发挥宽禁带半导体材料的优势,并提升宽禁带半导体电子器件的性能具有重要意义。得益于SiC衬底质量持续提升及成本不断降低,基于SiC衬底的宽禁带半导体电子市场占比呈现逐年增加的态势。在SiC衬底上外延生长高质量的宽禁带半导体材料是提高宽禁带半导体电子器件性能及可靠性的关键瓶颈。本文综述了近年来国内外研究者们在SiC衬底上外延SiC、氮化镓(GaN)、氧化镓(Ga₂O₃)所取得的研究进展,并展望了SiC衬底上宽禁带半导体外延的发展及应用前景。     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

I–V Characteristics of tris(2,4-pentanedionato)manganese(III) thin films

Amorphous thin films of tris(acetylacetonate)manganese(III) were deposited on Si(P) substrates by thermal sublimation in vacuum. The deposited films were probed with X-ray fluorescence. Their electrical properties were studied as insulators for Al/Mn(acac)₃/Si(P) metal–insulator–semiconductor devices. Those devices were characterized by the measurement of the gate-voltage dependence of their capacitance, from which the relative permittivity (RP) and density of the charges in the insulator were determined. It was found that values of the RP of tris(acetylacetonate)manganese(III) films grown on Si(P) wafers were in the range of 30–40, which can be find applications in gate related technological uses. The dc-electrical conduction in the complex film was studied at room temperature and in temperature range of (293–325 K). It was found that the data of the as-deposited films follow the trap-charge-limited space-charge-limited conductivity (TCL-SCLC) mechanism, while the data of the annealed sample in vacuum of about 10⁻³ Pa at about 100 °C for 10 min obey the Richardson–Schottky (RS) mechanism. The parameters of both mechanisms were determined. It was concluded that the density of charged defects in the insulating film is critically determined the mechanism of the current-transfer.     

A multi-wafer growth technique for GaAs vapor phase epitaxy using the AsCl3-Ga-N2 system with a horizontal reactor

A multi-wafer growth technique for vapor phase epitaxial GaAs has been developed using the open-tube AsCl₃-Ga-N₂ system with a conventional horizontal reactor. Use of this technique allows to process 4 wafers in a run with each wafer being 6 cm². The uniformities of growth rate, carrier concentration, and Hall mobility with wafer-to-wafer are typically ∽±3%, ∽±4%, and ∽±3%, respectively, and are shown to be sufficient for demanding microwave device applications.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

Optical properties of microcrystalline 3C–SiC:H films measured by resonant photothermal bending spectroscopy

Optical absorption coefficient spectra of hydrogenated microcrystalline cubic silicon carbide (μc-3C–SiC:H) films prepared by Hot-Wire CVD method have been estimated for the first time by resonant photothermal bending spectroscopy (resonant-PBS). The optical bandgap energy and its temperature coefficient of μc-3C–SiC:H film is found to be about 2.2 eV and 2.3 × 10⁻⁴ eV K⁻¹, respectively. The absorption coefficient spectra of localized states, which are related to grain boundaries, do not change by exposure of air and thermal annealing. The localized state of μc-3C–SiC:H has different properties for impurity incorporation compared with that of hydrogenated microcrystalline silicon (μc-Si:H) film.     

Infrared spectroscopy of Li-diborate glassy thin films

This work presents a structural investigation of Li-borate thin film electrolytes prepared by rf-sputtering from targets having the nominal Li₂O–2B₂O₃ composition. Thin films of ca. 1 μm were deposited on Si wafers and gold-covered Si substrates under argon, and their infrared spectra were measured in the 30–5000 cm⁻¹ range. The measured spectra of thin films were compared with those calculated on the basis of the infrared properties of the bulk Li-diborate glass and by considering all optical effects in the film/substrate system. The results showed that the thin films have the key structural features of the bulk glass, but exhibit also differences in the short-range order structure and in the Li ion-site interactions. These findings were discussed in terms of cooling rate differences between melt-quenching for bulk glass and sputtering for thin films.     

Recent achievements in AMMONO-bulk method

In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15 arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 10¹¹ Ω cm and p-type conductivity within hole concentration up to 10¹⁸ cm⁻³ are already available in diameters up to 1.5-in.     

Characterization of a Czochralski grown silicon crystal doped with 1020 cm‐3 germanium

A dislocation‐free silicon single crystal doped with 10²⁰ cm^‐3 germanium (Ge) has been grown using the Czochralski (CZ) growth technique. The Ge concentration in the seed‐end and tang‐end of the crystal was 8×10¹⁹cm^‐3and 1.6×10²⁰ cm^‐3, respectively. The effective segregation coefficient of Ge, the distribution of flow pattern defects (FPDs) and the wafer warpage have been characterized. Both the effective segregation coefficient and the equilibrium segregation coefficient of Ge in silicon were evaluated. Then, the density of FPDs was traced from seed‐end to tang‐end of the ingot, a suppression of FPDs by Ge doping was shown. That is probably because the Ge atoms consume free vacancies and thus a higher density of smaller voids is formed. Furthermore, the mechanical strength of wafers has also been characterized by batch warpage analysis. The warpage in the seed‐end was larger than that in the tang‐end of the ingot, showing that the mechanical strength of wafers is enhanced by Ge doping. Such improvement is interpreted by an enhanced dislocation pinning effect associated with the enhanced nucleation of grown‐in oxygen precipitates in the Ge‐doped silicon wafers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stacking faults in semi‐polar 6H‐SiC single crystals

6H‐SiC single crystals have been successfully grown on (1015) plane seed by sublimation method. High density stacking faults (SFs) were observed by transmission synchrotron radiation X‐ray topography. Based on the invisibility criteria of stacking faults, the displacement vectors of most SFs were determined to be the type of 1/6[1120]. Laser scanning confocal microscopy (LSCM) was used to observe the etching morphology of (0115) wafer. The etching steps of SFs were found and their density decreased from 3.6×10³ cm^‐1 to 2.0×10² cm^‐1 along the <0001> projection direction. The inclination angles of the SFs etching step plane to (10‐15) plane were measured by line scanning of LSCM. It was found that the inclination angles decreased from 20° to 10° along the <0001> projection direction. Different etching characteristics of SFs along radial direction of 6H‐SiC (1015) wafer should be attributed to different displacement vectors and different stacking fault energies for these stacking faults. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microstructure and optical properties of phase-change Ge–Sb–Te nanoparticles grown by pulsed-laser ablation

Phase-change Ge–Sb–Te (GST) nanoparticles have been in-situ synthesized by a pulsed-laser ablation method. During the ablation process, growth parameters including temperature of heat treatment, pressure, and laser fluence are extensively explored. Scanning and transmission electron microscopy are used to study microstructure and phase formation of the nanoparticles. Fourier transform analysis of electron micrographs exhibits an evidence of existence of the stoichiometric single GST-225 phase. We have measured micro-Raman scattering spectra of commercially available GST-124, -147, and -225 bulk and GST nanoparticles. Lack of the amorphous Te–Te stretching mode near 150 cm⁻¹ from the Raman spectra of the bulk samples indicates that the samples are well-crystallized. From the measurements of GST nanoparticles with different growth conditions, we could get information towards the optimal growth conditions for better crystalline quality of the GST nanoparticles. Our results suggest that micro-Raman scattering spectroscopy can be used to study phases and phase changes in the GST bulk crystals and nanoparticles through local structural information, which is being developed for low-power non-volatile memory applications.     

化学气相沉积法碳化硅外延设备技术进展

碳化硅(SiC)是制作高温、高频、大功率电子器件的理想电子材料,近20年来随着外延设备和工艺技术水平不断提升,外延膜生长速率和品质逐步提高,碳化硅在新能源汽车、光伏产业、高压输配线和智能电站等领域的应用需求越来越大。与硅半导体产业不同,碳化硅器件必须在外延膜上进行加工,因此碳化硅外延设备在整个产业链中占据承上启下的重要位置,而且也是整个产业链中最复杂、最难开发的设备。本文从碳化硅外延生长机理出发,结合反应室设计和材料科学的发展,介绍了化学气相沉积(CVD)法碳化硅外延设备反应室、加热系统和旋转系统等的技术进展,最后分析了CVD法碳化硅外延设备未来的研究重点和发展方向。     

Vapor growth and characterization of pyrite (FeS2) doped with Co,Ni, and As: Variations in semiconducting properties

Pyrite (FeS₂) crystals doped with As, Ni and Co were synthesized with chemical vapor transport over an 18 cm horizontal gradient of 700–600 °C in evacuated quartz tubes, from a mixture of FeS and S, with FeBr₃ as a transport agent. Sulfur fugacity and thus S:Fe stoichiometry was constrained by the Fe_1−xS/FeS_2−y buffer. As, Ni and Co concentrations were ∼3–800 ppm, ∼200–1500 ppm and ∼ 450–3700 ppm, respectively.Semiconducting properties were measured at room temperature using a van der Pauw and Hall measurement system. Ni and Co-doped pyrite are n-type while As-doped pyrite tends to be p-type. Resistivity for Co-doped pyrite ranged from 0.009 to 0.02 Ω cm while for Ni- and As-doped pyrite, resistivity ranged from 2 to 17 Ω cm. Undoped pyrite resistivity ranged from 15 to 85 Ω cm. Carrier concentration was similar for undoped and Ni-doped pyrite, ranging from 10¹⁵ to 10^16.6 cm⁻³, while for Co-doped pyrite it ranged from 10^18.7 to 10^19.3 cm⁻³ and for As-doped pyrite it ranged from 10¹⁴ to 10¹⁸ cm⁻³. Hall mobility was similar for Co and Ni-doped pyrite ranging from 60 to 270 cm² v⁻¹ s⁻¹ while for undoped pyrite it ranged from 8 to 70 cm² v⁻¹ s⁻¹. Hall mobility for As-doped pyrite ranged from 55.0 to 0.2 cm² v⁻¹ s⁻¹ for electrons and from 0.1 to 11.3 cm² v⁻¹ s⁻¹ for holes with the exception of one sample (of 22). These values should be viewed more as trends than as definitive. The results obtained for Ni, Co, and undoped pyrite are similar to those reported in the literature while results for As-doped synthetic pyrite have not previously been reported.     

Basic ammonothermal growth of Gallium Nitride – State of the art,challenges, perspectives

Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5?×?10⁴ cm^?2. Crystals with different types of conductivity, n-type with free electron concentration up to 10¹⁹ cm^?3, p-type with free hole concentration of 10¹⁶ cm^?3, and semi-insulating with resistivity exceeding 10¹¹ Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown.     

Investigation of 3C-SiC growth on Si(111) by vapor–liquid–solid transport using a SiGe liquid phase

The crystal growth of 3C-SiC onto silicon substrate by Vapor–Liquid–Solid (VLS) transport, where a SiGe liquid phase is fed with propane, has been investigated. Three sample configurations were used. In a preliminary approach, the VLS growth of SiC was conducted directly onto Si substrate using a Ge film as liquid catalyst. It led to the growth of a thick continuous SiC polycrystalline layer which was floating over a SiGe alloy located between the silicon substrate and the topping SiC layer. In the second configuration, a thin seeding layer of 3C-SiC grown by chemical vapor deposition (CVD) was used and the VLS growth was localized using a SiO₂ mask. The liquid phase was a CVD deposited SiGe alloy. The growth of a few hundred nanometers thick 3C-SiC epitaxial layer was demonstrated but the process was apparently affected by the presence of the oxide which was dramatically etched at the end. In the last configuration, the silicon substrate was patterned down to 10 μm and a thin seeding layer of 3C-SiC was grown by CVD onto this patterned substrate. The liquid phase was again a CVD deposited SiGe alloy. In this last configuration, the presence of epitaxial SiC was evidenced but it grew as trapezoidal islands instead of an uniform layer.     

Preparation of Ge-Si epitaxial alloys by sputtering

Ge-Si alloy layers have been epitaxially grown throughout the whole range of composition onto Ge substrates by the simultaneous getter sputtering from elemental Ge and Si sources. The epitaxial temperature was 550 to 830° C at growth rates of about 1 μm/h, depending on the Si atomic fraction in the range of 0.05 to 0.88. As the Si content in the alloy increases, the crystallinity of the layer decreases: Si-rich alloy layers contained microtwins. Hall measurements of alloy layers without intentional doping indicated p-type conductivity with Hall mobility of 600 cm²/V·sec at carrier concentration of 2 × 10¹⁶ cm^-3 for 25 at% Si in the alloy at room temperature. The observed temperature dependence of the hole mobility is indicative of alloy scattering.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.     

Superplastic flow of a Mg-based bulk metallic glass in the supercooled liquid region

The tensile flow behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated over a range of strain rates (10⁻³–10⁻¹/s) and deformation temperatures (150–170 °C) in the supercooled liquid region. In this region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars–Tegart constitutive relationship. There was also a good correlation between the Zener–Hollomon parameter, Z, and the flow characteristics of the material such as the transition from Newtonian to non-Newtonian flow and maximum achievable tensile elongation; the latter was used for determining the optimum conditions for superplastic flow in the material.     

Er doping of Si and Si0.88Ge0.12 using Er2O3 and ErF3 evaporation during molecular beam epitaxy A transmission electron microscopy study

The incorporation behavior of Er into Si and Si_0.88Ge_0.12 using ErF₃ and Er₂O₃ as dopant sources during molecular beam epitaxy has been studied. The Er-compounds were thermally evaporated from a high-temperature source. Dissociation of Er₂O₃ took place and reaction with graphite parts in the high temperature source gave an increased CO background pressure and evaporation of metallic Er. Surface segregation of Er may be strong, but with a high CO or F background pressure, the surface segregation could be reduced and sharp Er concentration profiles were obtained. Transmission electron microscopy analysis shows that it is possible to prepare high crystalline quality structures with Er concentrations up to 4×10¹⁹ cm⁻³ using Er₂O₃ and a high F background pressure. Using ErF₃ compound as source material a F/Er incorporation ratio of approximately three has been measured by secondary ion mass spectrometry. Fluorine incorporation can occur not only from evaporated units of ErF₃ molecules, but also from CF_x (x=1–4) and F background species, which are present due to a reaction between the ErF₃ source material and the graphite crucible in the source. After careful degassing of the source, the partial pressures of these species can be significantly reduced. By producing an Er-doped multilayer structure consisting of alternating doped layers grown at low temperature (350°C) and undoped layers grown at a higher temperature (630°C), a flat surface could be maintained during the growth sequence. In this way it was possible to prepare Er-doped structures with an average Er concentration of 1×10¹⁹ cm⁻³ and without observable defects using ErF₃ as source material. For the case of Er-doping of Si_0.88Ge_0.12 using ErF₃, we observed contrast along lines in the growth direction at an Er concentration of 1×10¹⁹ cm⁻³, which was attributed to Si concentration variations. Intense emission related to Er has been observed by electro- and photoluminescence.